A high-precision A/D converter such as a delta-sigma (ΔΣ) type A/D converter is used to detect a battery voltage. The ΔΣ-type converter necessitates over-sampling. For this reason, a capacitor is charged and discharged repetitively at a relatively short period and hence a leak current flows out from the battery, the voltage of which is to be detected. A system for detecting the battery voltage is provided with a low-pass filter at its input stage. The low-pass filter is formed of a capacitor and a resistor having a relatively high resistance value. The leak current from the battery flows to the resistor of the filter and causes a voltage drop. This voltage drop is not negligible relative to a detection value of the voltage and causes a detection error.
US 2012/0274360A therefore proposes a compensating circuit (referred to as a leak canceling circuit below), which generates a compensating current flowing in opposition to the leak current. The compensating current cancels out the leak current thereby to reduce the detection error caused by the voltage drop generated by the resistor in the filter.
In case that the leak canceling circuit fails to operate property, not only the detection precision is lowered but also the detection value may deviate very much from a normal detection case in a worst case. Such a deviation in the detection value will tend to cause erroneous operations in various processing, which use the voltage detection value.